1. Field of the Invention
The present invention relates, in general, to PbZr.sub.x Ti.sub.1-x O.sub.3 (hereinafter referred to as "PZT") thin films for ferroelectric capacitors and, more particularly, to introduction of dopants for improvement in endurance properties and electrical properties of PZT films. The present invention is also concerned with methods for preparing the PZT ferroelectric thin films.
2. Description of the Prior Art
Of the ferroelectric materials used for capacitors, PZT exhibits exceptionally high polarization values and superiority in electrical and material properties. PZT, widely utilized for applications in ferroelectric devices, has a perovskite structure in which a phase transition between a tetragonal phase and a rhombohedral phase appears when the composition ratio of Zr to Ti is 53:47. Around this morphotropical phase boundary, PZT exhibits superior properties as a ferroelectric material with maximized polarization and dielectric constant.
A significant disadvantage of PZT as a ferroelectric material is in its inferior endurance. For example, when subjected to repeated switching, PZT shows a deterioration in polarization properties, such as reduction of remanent polarization and a distortion in the shape of the hysteresis loop. This phenomenon is known as fatigue.
In order to utilize ferroelectric materials as memory devices, these materials are required to have an endurance of 10.sup.12 cycles or more. In general, after 10.sup.6 cycles, pure PZT begins to show serious fatigue. After 10.sup.9 cycles, it no longer exhibits a ferroelectric property or the material itself is broken down.
To date, various efforts have been made to improve the endurance of PZT thin films for ferroelectric capacitors.
First, there was an attempt to change the electrode material from conventional Pt to a material selected from metal oxides with relatively good electroconductivity. It has been accepted that one of the known fatigue mechanisms for PZT thin film is the accumulation of an oxygen vacancy at the interface between the PZT thin film and the electrode in the ferroelectric capacitor. That is to say, oxygen vacancies randomly generated in the PZT thin film by repeated switching cycles are gradually accumulated at the interface between the PZT thin film and the electrode. The oxygen-vacant layer is expanded with the repetition of switching cycles, and finally the entire PZT thin film becomes electrically degraded. It has been recognized that a metal oxide electrode material can substantially accommodate the oxygen vacancies accumulated at the interface between the electrode and the PZT thin film and thus, the endurance of the PZT thin film can be significantly improved. As an example, it was reported that the electrode endurance cycle was notably improved by employing RuO.sub.2 as an electrode material (See, D. P. Vijay, C. K. Kwok, W. Pan, I. K. Yoo and S. B. Desu, "Electrode Effects on Electrical Properties of Ferroelectric Thin Films", ISAF Proceedings 8th IEEE, 408 (1992)).
However, the use of a conductive metal oxide an electrode material results in an increase in the leakage current in a PZT ferroelectric capacitor. In addition, another problem is that the fabrication process for an electrode of a conductive metal oxide is too intricate to yield an electrode with good reproducibility.
Another attempt to improve the endurance of PZT thin films for ferroelectric capacitors was made by doping PZT with donors in order to reduce the oxygen vacancies in the PZT thin film. For example, La was employed as the dopant. However, PZT doped with La proved to be an unsuitable system because the remanent polarization rapidly decreased as the amount of the dopant increased. PZT was doped with other donor elements, Nd and Nb, by S. B. Desu et al. However, they could not achieve any appreciable improvement in the fatigue of the PZT films.